add support for aborting via AbortController

https://developers.google.com/web/updates/2017/09/abortable-fetch

Currently it is only implemented in Firefox 57 and is coming to other browsers soon.

```javascript
const controller = new AbortController();
const signal = controller.signal;

setTimeout(() => controller.abort(), 5000);

fetch(url, { signal }).then(response => {
return response.text();
}).then(text => {
console.log(text);
});
```

Will most likely need to implement another npm package that just include `AbortController` package. I found one but it also pollyfills fetch. https://github.com/mo/abortcontroller-polyfill